474 THE POPULAR SCIENCE MONTHLY 
distinct, at first sporadically along with the more primitive ones, then 
more abundantly, finally replacing the older ones altogether. The 
intermediate gradations occur along with the more typical individuals, 
but without much definite relationship to intergradation in the succes- 
sion of strata.? 
We may illustrate from the evolution of the oreodonts, as these are 
the most abundant and most completely known of American fossil 
mammals. 
The earliest known representatives of the phylum are Protoreodon 
and Protagriocherus from the Upper Eocene Uinta beds of Utah. Both 
have very short crowned teeth with five crescents on the upper molar, the 
fifth crescent quite distinct. The fourth premolar is not molariform. 
For the next stage we have to shift to another formation, 400 miles 
away, the White River. In the lowest strata of this formation, the 
Titanotherium beds, we find Oreodon, Bathygenys and Agriocherus, 
all with decidedly longer crowned teeth, and no trace of the fifth cres- 
cent in the molars. In Oreodon and Bathygenys the fourth premolar 
is non-molariform, composed of one inner and one outer crescent, as 
usual among Artiodactyls. In Agriochwrus it has become imperfectly 
molariform with two outer crescents and one inner one. Between the 
Uinta and White River oreodonts a sharp break intervenes and no 
intermediates are known. From this point we can trace the subphyla 
of oreodonts up through a considerable succession in the Big Badlands 
of South Dakota and the adjoining region. Oreodon culbertsom, O. 
bullatus, Hucrotaphus, Eporeodon, Mesoreodon and Merychyus appear 
to be approximately successive stages in specialization. The skull is 
shortened, the teeth become longer crowned, the tympanic bulle are 
enlarged, lachrymal vacuities appear, the limbs are lengthened, the 
feet lengthened and compacted and the thumb is lost. But there is 
not a continuous intergradation in any of these features as we pass 
upward in the beds. Oreodons with small bulle are abundant in the 
lower and middle White River, the bulle varying very little in size. 
A species with medium-sized bulle occurs occasionally associated with 
them. In the Upper White River all the oreodons that I have seen have 
bulle of large size. The size of the bulla, then, does not increase con- 
tinuously as we go up through the formation. Another and much 
more specialized genus of oreodonts, Leptauchenia, suddenly appears 
in abundance in the Upper White River. I have seen a single specimen 
of this genus from the Middle beds, but it shows no more primitive 
features than those of the Upper beds. In the Lower Rosebud, immedi- 
ately overlying the White River, species of Eporeodon are common, like 
The statements of fact herein contained are based partly upon field experi- 
ence, chiefly upon the records of some 20,000 specimens of fossil mammals and 
reptiles in the American Museum collections, most of which the writer has had 
occasion to examine and identify and to post the field records of level and 
locality, in the course of cataloguing work. 
